The present invention relates to a mass spectrometer and, more particularly, to a mass spectrometer suitable for a mass spectrometer of ion trap type.
As shown in FIG. 2, a mass spectrometer of ion trap type comprises a ring electrode 6 having a ring shape, two end-cap electrodes 7 and 8 arranged in the axial direction (z-axis) of the ring electrode 6 so as to sandwich the ring electrode 6. By applying a direct current voltage U and a radio frequency (R.F.) voltage V cos(.OMEGA.t) to each of these electrodes, a quadrupole electric field is formed in a volume among the electrodes. Stability of an ion trajectory in the quadrupole electric field is determined by an a-value and a q-value expressed by Equation 1. EQU a=8ZeU/(mr.sub.0.sup.2 .OMEGA..sup.2), q=4ZeV/(mr.sub.0.sup.2 .OMEGA..sup.2)(1)
where e is the quantum of electricity, r.sub.0 is the internal radius of the ring electrode 6, m is an ion mass, Z is an ionic charge number, .OMEGA. is an angular frequency of the radio frequency voltage, U is a direct current (DC) voltage and V is an amplitude of the radio frequency voltage.
FIG.3 shows a stability diagram expressing the range of a- and q-values which determine the stability of an ion trajectory inside the ion trap. A group of curves shown inside the stability region are iso-.beta. lines of parameters, .beta.r and .beta.z, which define oscillation characteristics of ions in the r-direction (the radial direction of the ring electrode 6) and the z-direction (the axial direction of the ring electrode 6). The stability region corresponds to a region surrounded by the lines, .beta.r=0, .beta.r=1.0, .beta.z=o and .beta.z=1.0. Ions tracing a stable trajectory correspond to a point inside the stability region depending on an m/Z (mass-to-charge ratio) of the ions. Therefore, values .beta.r and .beta.z are determined corresponding to the m/Z.
The oscillating movement of an ion can be decomposed into components of r (radial) and z (axial) directions. The oscillation characteristics of the r- and z-directions are determined by the values of .beta.r and .beta.z, respectively. Fundamental angular frequencies .beta.r and .beta.z, known as secular angular frequencies, determining oscillation in the r- and z-directions are given by Equation 2. EQU .omega.r=.OMEGA..beta.r/2, .omega.z=.OMEGA..beta.z/2, (2)
In a conventional resonance ejection method, as shown in FIG. 7, alternating voltages .DELTA..phi..sub.1 (=V.sub.0 cos(.omega.z t) and .DELTA..phi..sub.2 (=-.DELTA..phi..sub.1) having the same angular frequency as the fundamental angular frequency of the oscillation in the z-direction is applied to the end cap electrodes 7 and 8. Since the pointing direction of the z-direction component of the auxiliary electric field E.sub.0 generated between the end cap electrodes by the alternating current voltage is alternatingly changed, the ion trajectory becomes unstable equally to both the positive and negative sides of the z-direction. For example, in a case where a mass spectrometer is connected to a gas chromatograph, it is required to arrange an electron gun 5 for producing ions in the side of one end cap electrode 7 in order to ionize a sample in the volume surrounded by the three ion trap electrodes, and a detector 9 cannot be chosen but arranged in the side of the other end cap electrode 8, as shown in FIG. 2. In this case, since the ion trajectory becomes unstable to both of the positive and negative sides of the z-axis, ions unstabilized in the side of the electron gun 5 are not detected.
Examples of such conventional technologies are disclosed in Japanese Patent Application Laid-Open No. 1-258353 (1989) where ions are detected while the trajectory is unstabilized after one ion species is stably trapped, and both in Japanese Patent Application Laid-Open No. 63-313460 (1988) and Japanese Patent Application Laid-Open No. 2-103856 (1990) where ions are resonance ejected in the z-direction to identify the mass by applying an ion exciting voltage having an oscillation frequency equal to a secular frequency of trapped ions between end cap electrodes.
In the above conventional technologies, the detecting efficiency is low since ions unstabilized to the side of the electron gun cannot be detected though the ion trajectory is unstabilized to both the positive and negative sides of the z-axis (that is, the pointing directions in which the ions are unstabilized are the positive side and the negative side of the z-axis).
In addition to the above, the conventional mass scanning method requires a very long time to analyze a whole range of all ion species, and there is possibility to degrade the analyzing accuracy due to occurrence of displacement in mass spectrums (mass shift) when the ions are trapped inside an ion trap for a long time. In other words, with a conventional method shown in FIG. 13, an amplitude of a radio frequency voltage is linearly varied with time while a resonance voltage having a constant amplitude is applied throughout the whole scanning of mass-to-charge ratios within the interest range of M.sub.1 to M.sub.n. Therefore, in the conventional method, as the value of a mass-to-charge ratio increases, the amplitude ratio of an auxiliary voltage and a radio frequency voltage decreases, and consequently the higher a mass-to-charge ratio of ion becomes, the higher the mass resolution becomes. However, since the amplitude of radio frequency voltage is scanned linearly to time, analyzing time distributed to each ion species is nearly equal. Therefore, if the amplitude ratio of the auxiliary voltage to the radio frequency voltage is set to such a small value that an ion species having a high mass number can be analyzed with a target mass resolution, it is necessary to allocate an analyzing time period sufficient enough to unstabilize the ions having the high mass number to all ion species, which increases the total analyzing time. That is, a long analyzing time period is allocated even to an ion having a small mass number which requires a small ratio of an auxiliary voltage V.sub.a to a radio frequency voltage V and an unstabilizing time shorter than that required for a high mass number ion.
In the method described above, since the mass-to-charge ratios for interest ions are scanned at a constant speed, a constant time period is allocated to analyzing each ion species having different mass-to-charge ratios. Therefore, if the amplitude ratio of the auxiliary voltage to the radio frequency voltage is set to such a small value as to match with an ion species having a high mass-to-charge ratio and requiring a high resolution, it takes a very long time to unstabilize the trajectory for an interest ion species. That is, a constant very long analyzing time is allocated to analysis of each ion species. Further, in mass spectrometry of the above method, it requires a very long time to analyze all ion species within the range of interest masses, and particularly the amplitude ratio is set too high for an ion having a low mass-to-charge ratio and an unnecessary long time is spent even for analyzing an ion which can be unstabilized in a short time.
On the other hand, if ions are contained inside an ion trap for an unnecessary long time, the ions are subjected to collisions with a neutral gas or large interactions with the other ions. Therefore, there is possibility to degrade the analyzing accuracy due to occurrence of displacement in mass spectrums (mass shift).